Increasing attention has been paid to the dangers of microorganism contamination from everyday exposures. While primarily a concern for a wide range of healthcare facilities, and food processing and preparation facilities, it is also a concern for schools, public transport, the home and businesses. Healthcare facilities where microbial bio-burden is a concern include large multi-unit hospitals, specialized clinics, veteran affairs hospitals, long term care facilities, retirement homes and individual or group doctors or dental offices among others.
Drug resistant strains of pathogenic bacteria are being identified around the world, and the spread of these microorganisms from local to regional to worldwide is well documented. New microorganisms, or more virulent forms of existing micro-organisms, especially antibiotic resistant strains, are also being discovered, and can readily spread worldwide due to the growing ease of travel, and the developing worldwide market for goods. Microorganisms of concern include but are not limited to bacteria of all sorts, fungi, parasites, and many types of viruses. Although regular cleaning and good sanitation practices can be effective means of reducing microbial bio-burden, it would be beneficial to provide materials which are inherently resistant to, or minimize the spread of microorganisms.
The antimicrobial properties of silver have been known for a very long time. The pharmacological properties of silver are described in “Heavy Metals” by S. C. Harvey and in “Antiseptics and Disinfectants: Fungicides; Ectoparasiticides by S. Harvey in The Pharmacological Basis of Therapeutics, Fifth Edition, Goodman and Gilman (editors), MacMillan Publishing Company, NY, 1975. The mechanism of action of silver has also been described by Clement and Jarrett in Metabolism Based Drugs 1(5-6), 467-482. Some basic mechanisms of action which have been identified include degradation of bacterial enzymes, cell wall degradation, inhibition of cell mitotic activity, degradation of cytoplasmic structures, and interaction with DNA bases. It is recognized in the literature that both metallic silver and silver ions are antimicrobial, but that ultimately antimicrobial activity is mediated through the dissolution of silver ions into the bacterial microenvironment.
Silver sulfate is a well-known, commercially available material that can be synthesized by conventional aqueous precipitation methods. The reaction of aqueous solutions of silver nitrate and sulfuric acid to form silver sulfate was described by Richards and Jones in Z. anorg. Allg. Chem 55, 72, (1907), and an improvement on the method was published by Hahn and Gilbert Z. anorg. Allg. Chem 258, 91, (1949). Silver salts are generally known to be thermally and photochemically unstable, forming brown, gray or black products. Silver sulfate may be reduced to its metallic state, with the corresponding oxidation of chemical elements in its environment. It can also be converted to or converted to silver oxide (black) or silver sulfide (black) by exposure to air. Silver metal generated by thermal reduction on a polymeric substrate will exhibit a UV absorption band at 390 nm which is attributable to the surface plasmon resonance of silver.
One use of silver based antimicrobials is for textiles. Various methods are known in the art to introduce antimicrobial properties to a target fiber. The approach of embedding inorganic antimicrobial agents, such as zeolites, into low melting components of a conjugated fiber is described in U.S. Pat. Nos. 4,525,410, and 5,064,599. In another approach, the antimicrobial agent can be delivered during the process of making a synthetic fiber such as those described in U.S. Pat. Nos. 5,180,402, 5,880,044, and 5,888,526, or via a melt extrusion process as described in U.S. Pat. Nos. 6,479,144 and 6,585,843. In yet another process, an antimicrobial metal ion can be ion-exchanged with an ion-exchange fiber as described in U.S. Pat. No. 5,496,860. High-pressure laminates containing antimicrobial inorganic metal compounds are disclosed in U.S. Pat. No. 6,248,342. Deposition of antimicrobial metals or metal-containing compounds onto a resin film or fiber has also been described in U.S. Pat. Nos. 6,274,519 and 6,436,420. An antimicrobial mixture of zinc oxide and silver sulfate on an inorganic powder support is disclosed in JP 08133918. An antimicrobial masterbatch formulation is disclosed in JP 2841115B2 wherein a silver salt and an organic antifungal agent are combined in a low melting wax to form a masterbatch with improved mixing and handling safety. More specifically, silver sulfate was sieved through a 100 mesh screen (particles sizes less than about 149 microns), combined with 2-(4-thiazolyl)benzimidazole and kneaded into polyethylene wax. This masterbatch material was then compounded into polypropylene, which was subsequently injection molded into thin test blocks which exhibited antibacterial properties with respect to E. coli and Staphylococcus, and antifungal properties with respect to Aspergillus niger. Similar masterbatches are also described in JP 03271208, wherein a resin discoloration-preventing agent (e.g. UV light absorbent, UV light stabilizer, antioxidant) is also incorporated.
Silver sulfate has been used as an antimicrobial agent in multiple medical applications. Incorporation of inorganic silver compounds in bone cement to reduce the risk of post-operative infection following the insertion of endoprosthetic orthopaedic implants was proposed and studied by J. A. Spadaro et al (Clinical Orthopaedics and Related Research, 143, 266-270, 1979). Silver chloride, silver oxide, silver sulphate and silver phosphate were incorporated in polymethylmethacrylate bone cement at 0.5% concentration and shown to significantly inhibit the bacterial growth of Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. Antimicrobial wound dressings are disclosed in U.S. Pat. No. 4,728,323; wherein a substrate is coated with an antimicrobially effective film of a silver salt, preferably silver chloride or silver sulfate. Antimicrobial wound dressings are disclosed in WO2006113052A2, wherein aqueous silver sulfate solutions are dried onto a substrate under controlled conditions to an initial color, which is color stable for preferably one week under ambient light and humidity conditions. An antimicrobial fitting for a catheter is disclosed in U.S. Pat. No. 5,049,140 which describes a tubular member composed of a silicone/polyurethane elastomer in which is uniformly dispersed about 1 to 15% wt. of an antimicrobial agent, preferably silver sulfate. A moldable plastic composite comprising cellulose and a urea/formaldehyde resin is disclosed in WO 2005/080488A1, wherein a silver salt, specifically silver sulfate, is incorporated to provide a surface having antiviral activity.
Despite various references to the proposed use of silver salts as antimicrobial agents in various polymers as referenced above, there is little or no disclosure in the art of methods for preparing silver-containing materials comprising higher melting polymers such as polyethylene terephthalate (PET) polyester. Even the relatively thermally stable silver sulfate salt is converted to metallic silver when heated excessively in an organic matrix at the temperatures necessary to melt and extrude PET, for example in processes for making polyester (e.g., PET) fiber. Other silver salts such as the silver halides and silver nitrate are even more thermally sensitive than silver sulfate and are thus more prone to reduction to metallic silver if processed at high temperature, for example under typical polyester fiber production process conditions.
In order to avoid problems with the thermal instability of silver salts during fiber production, various processes have been proposed in which different dispersions of silver salts in multi-component mixtures have been applied to fabrics of different compositions as topical agents after the fiber has been extruded, or after the fabric has been constructed. However, fabrics in which e.g. silver salts have been topically applied exhibit poor laundering properties and leaching of the topical silver salt from the topical fabric coating. It is also difficult to control the amount of silver salt topically absorbed onto the surface of the fiber or the fabric because of the various finishing solutions, dyes flame retardants or other agents commonly applied to fibers or textiles. Topical coatings on textile grade fibers or fabrics may have utility in certain settings, but are viewed as a fundamentally different technology relative to a textile grade polyester fiber having the antimicrobial agent incorporated into the fiber during the fiber extrusion process.
In addition, the use of metallic silver nanoparticles as antimicrobial and antifungal agents in textiles has been attempted but has generally been unsuccessful due to problems with clumping and other challenges, and it is been difficult to obtain a controlled, uniform dispersion and concentration of the metallic silver nanoparticles in the final textile product. Theoretically, the high surface area of the metallic silver nanoparticles offers an advantage over micron sized metallic silver particles as antimicrobial and/or antifungal agents due to the nature of the ion release mechanism. The release mechanism involves water or oxygen mediated oxidation and dissolution of silver ions from the silver metal surface, which occurs in proportion to the surface area. However, metallic silver nanoparticles are difficult to incorporate into textiles to produce a product having desirable properties including durable antibacterial and/or antifungal biocidal activity after repeated use and washings.
A method for incorporating metallic silver nanoparticles into polyester and other synthetic polymeric fibers is disclosed in U.S. Pat. No. 8,183,167. However, this technology suffers from the disadvantage that metallic silver nanoparticles release silver ions more slowly than particles of silver salts such as silver sulfate. In addition, it is recognized in the literature that metallic silver nanoparticles can be absorbed into cells directly and have their own toxicity characteristics separate from that of silver ions. Incorporation of metallic silver nanoparticles into the fiber during polymer extrusion, or topically treating fibers or fabrics with metallic silver nanoparticles after extrusion, is considered a very different technology compared to incorporating silver salt particles into the extruded fiber.
Foss et al. in U.S. Pat. Nos. 6,723,428, 6,841,244, and 6,946,196 disclosed multilayer and multicomponent antimicrobial fabrics and articles employing preferably silver-containing zeolites as antimicrobial agents, for example in a thin “shell” layer on the exterior of the fiber, in a relatively low melting carrier polymer (e.g. PETG), or in a latex which is used as a vehicle for impregnating e.g. a shoe insole. When a low melting polymer carrier is used, the fabric containing the low melting polymer and incorporated antimicrobial agent is heat activated to melt the polymer and disperse the antimicrobial agent throughout the fabric.